This invention relates to an electrosurgery system, an electrosurgical instrument, and a method of performing electrosurgery.
The cutting or removal of tissue electrosurgically using an instrument having a tip with one or more active electrodes supplied with a radio frequency voltage usually involves cell rupture as a result of arcs between the active electrode and the tissue being treated or, in the case of underwater electrosurgery, between the active electrode or electrodes and a conductive liquid overlying the tissue to be treated. As described in European Patent Application No. 0754437A, electrode destruction can occur if sufficient radio frequency power is supplied to an electrode to cause burning or melting of the electrode material, and this can be avoided by sensing peak electrode voltage and applying feedback to reduce the applied power so as to set a maximum peak voltage. It will be understood that for a given power setting, the temperature of the electrode depends on the rate at which heat can be dissipated which, in turn, depends on such variables as the degree of tissue engagement, electrode structure, and fluid flow around the electrode. Consequently, to avoid electrode destruction the peak voltage limit must be set at a sufficiently low level to prevent damage in the worst case dissipation situations, i.e. when there is an absence of cooling fluid and/or the electrode is surrounded by tissue.
It is known that the arcing at the electrode can cause nerve stimulation. This is due to the thermionic effect when the electrode is heated. Since the use of a coupling capacitor between the radio frequency source in the generator and the electrode is mandatory, the electrode is isolated, allowing charge to build up on the capacitor, which produces nerve stimulation when the electrode touches tissue or conductive fluid overlying the tissue.
The invention is based on the recognition that the thermionic effect is temperature dependent and, therefore, may be used as a indication of electrode temperature.
Accordingly, the invention provides an electrosurgery system comprising a generator which includes a source of radio frequency energy for feeding to a treatment electrode of an electrosurgical instrument, wherein the generator includes a circuit for sensing thermionic emission from the electrode.
The invention also includes an electrosurgery system comprising a generator including a source of radio frequency (r.f.) energy and, coupled to the generator, an electrosurgical instrument having a treatment electrode, wherein the system includes a d.c. voltage detector arranged to detect a d.c. offset in excess of 2 volts on the treatment electrode, and, coupled to the detector, a control circuit for controlling the r.f. energy applied to the instrument in response to the d.c. offset. In this way, the level of the d.c. voltage at the treatment electrode due the thermionic effect may be limited as a means, for instance, of controlling electrode temperature. Indeed the association of the thermionic effect with arcing and, in particular, arcing at such a level and in such surroundings that the treatment electrode becomes very hot, can be used in accordance with the invention for electrode overheat protection. This, in turn, allows an electrode used in surroundings having other than worst case heat dissipation to be operated at a higher temperature than would be possible using the prior art voltage-sensing technique, with a consequent increase in the rate at which tissue may be cut or removed.
Typically, for overheat protection, the control circuit and detector are operable to limit the d.c. offset to a predetermined d.c. voltage level in the region of from 50 V to 100 V. In practice, the actual voltage level depends on electrode configuration and electrode material. Thus, if a platinum electrode is used, the voltage limit is set to that which occurs when the electrode voltage approaches 1600xc2x0 C., the melting point of platinum.
Alternatively, the maximum d.c. offset voltage may be set to a much lower value to reduce nerve stimulation in sensitive sites such as in neurosurgery, surgery in the region of the spine, and in muscle tissue.
In a preferred embodiment of the invention, the generator has an output terminal connectible to the treatment electrode and the detector has (i) a detection input which is connected to the output terminal and (ii) an isolation device connecting the detector to the control circuit. The detector may be powered from the generator r.f. output energy by having a power supply circuit coupled to the generator output terminal and including a rectifier for rectifying the r.f. electrosurgery signal applied to the output terminal. This is permissible since the thermionic effect does not occur until the r.f. output voltage reaches a level consistent with arcing. The fact that the detector does not function at lower voltages is, as a result, no disadvantage. Typically, to achieve isolation at the output of the detector, it comprises an oscillator for generating an alternating measurement signal representative of the d.c. offset, and the isolation device comprises an opto-isolator coupled to receive the alternating measurement signal and to feed it to the control circuit. The preferred detector also includes a reverse polarity d.c. offset detector as a fault condition indicator which can be used to disable the r.f. source when, for instance, in use of a bipolar electrode assembly in a conductive fluid field, a lack of fluid causes d.c. polarity reversal.
According to a second aspect of the invention, an electrosurgical generator including a source of r.f. energy, an active output terminal, a return output terminal and a d.c. isolation capacitor between the source and the active output terminal, has a d.c. offset detector having a shunt input connected on the output terminal side of the isolation capacitor for detecting d.c. voltages in excess of 2 volts, and a control circuit coupled to the source for controlling the r.f. energy applied to the active output terminal in response to the d.c. offset voltage at the shunt input.
According to a third aspect of the invention, a method of performing electrosurgical tissue cutting or ablation comprises applying r.f. energy to an electrosurgical instrument so as to promote arcing at a treatment electrode of the instrument, and regulating the level of applied energy according to the d.c. voltage attained at the electrode due to thermionic emission from the electrode. The r.f. energy may be regulated to maximise the temperature of the electrode without substantial electrode damage, typically by limiting the d.c. voltage to a threshold value of less than 100 V.
In the above-described preferred embodiment, detection of the thermionic effect occurs as a result of the continual discharge of capacitors in the output stage of the generator through a high value resistance, the detected DC voltage across the electrodes being representative of this effect. Since this detection technique is limited by the time constant for the discharge of the capacitors in the output stage, it has the disadvantage of having a relatively slow response time. The time constant is the product of the capacitance and resistance in the circuit. The capacitors are necessarily large to reduce output impedance, and the shunt resistance connected across the output terminals is large to prevent the flow of significant direct current through the patient.
A further embodiment provides an alternative technique for detecting thermionic emission from the active electrode in which an alternating current phenomenon affected by thermionic emissions is detected, obviating the need for a DC conducting path across the output terminals of the generator. Accordingly, a further aspect of the present invention provides a modulated generator output, and a detection circuit coupled to the generator output by means of a coupling which does not include any DC conductive path, the detection circuit being tuned to the frequency of the modulation of the generator output, wherein thermionic emissions from the active electrode cause manifestation of the modulation in the generator output in the detection circuit.
In a preferred embodiment the amplitude of the generator output is modulated in a manner such that, in the absence of any thermionic effect, the average value of the generator output as measured over a single cycle of the generator output remains constant; the existence of thermionic emissions causes the average value of the generator output measured over a single cycle to vary at the frequency of the generator amplitude modulation, and the amplitude of this variation in average value corresponds to the extent of the thermionic effect. The detection circuit may be provided by a suitably tuned resonant circuit and a detector which is inductively coupled to the generator output stage to detect a voltage in the resonant circuit which results from thermionic emission.